[Rob. Smith]

My mates' cobra is a DRB. It's got SLR 5000 brakes, discs on the front and drums on the back. The problem is the proportioning valve. He had it rebuilt and was told that parts aren't available so they put it back together with "O" rings. It blew out the other day and lost most of the fluid. I showed him the set-up on my RMC. Ford master with built in proportioning valve and sigma booster all operating jag brakes.
My question is...is there any other master cylinder / booster set-up suitable for a DRB with drum / disc arrangement ?
Any advice would be good. Smithy